1. Field of the Invention
This invention relates to the structure of a buckle made of synthetic resin for a strap used in a wrist watch or the like.
2. Prior Art
Synthetic resin has recently come to be used as the material for the buckle body of a strap since it is highly economical which results from mass productivity and has resilience which is helpful during assembly. The buckle body usually has a generally C-shaped configuration having the two leg portions. Each of the leg portions has a recess or hole at end portion thereof. A spring rod having a slider pushed by spring so as to project in an outward direction or a pin is set between the leg portions of the buckle body by inserting the slider into the recesses or holes of the leg portions. However, the buckle body has little rigidity since it is made of a synthetic resin, so that the spring rod or pin readily slips out from between the two leg portions of the buckle body when these two leg portions are spread apart by an external force.
To eliminate this shortcoming, it has been propoased as illustrated in Japanese Utility Model Application Laid-Open No. 58-80022 to press fit a metallic mounting pin into one hole of the C-shaped buckle body so that the mounting pin will not slip out of the buckle body.
A prior-art example of a strap buckle using this press-fitted arrangement is shown in the perspective view of FIG. 5 and the plan view of FIG. 6. A generally C-shaped buckle body 10 is made of a synthetic resin material and includes opposing first and second leg portions 12, 14 including first and second stepped holes 16, 18 formed at their end portions, respectively, having a stepped cross section. These first and second holes 16, 18 each have a small diameter on the inner side (sides of opposing faces of the first and second leg portions 12, 14) and a larger diameter on the outer side (sides of outer faces of the first and second leg portions 12, 14).
A mounting pin 20 for connecting the first leg portion 12 and second leg portion 14 has one end thereof formed to include a small-diameter neck portion 22 and a large-diameter locking projection 24 provided at the tip of the neck portion 22. The locking projection 24 has an outer peripheral side surface which tapers off to the tip of the projection and the maximum diameter being made smaller than the small-diameter portion of stepped first hole 16. The other end of mounting pin 20 is formed to include a head 26 having a diameter larger than the small-diameter portion of the stepped second hole 18. The small-diameter portion of the stepped second hole 18 is sized to allow the locking projection 24 as well as a main portion 28 of the mounting pin 20 to be passed therethrough.
When the first leg portion 12 and second leg portion 14 of the buckle body 10 are connected by the mounting pin 20, the locking projection 24 of the mounting pin 20 is passed through the stepped second hole 18 toward the side having the stepped first hole 16. Then the mounting pin 20 is passed through an insertion hole 32 of a strap 30 and a looped portion 36 of a tongue 34. After the locking projection 24 reaches to the entrance to the stepped first hole 16, the mounting pin 20 is pushed forwardly to connect the locking projection 24 to the step portion of the stepped first hole 16. Though the maximum diameter of the locking projection 24 is greater than the small-diameter portion of stepped first hole 16, owing to resilience of the buckle body 10 consisting of the synthetic resin, the locking projection 24 passes through the small-diameter portion of first hole 16 and reaches to the large-diameter portion. With the locking projection 24 thus press fitted into the stepped first hole 16, the head 26 at the other end of mounting pin 20 reaches to the step of the second hole 18.
Thus, the arrangement is such that locking projection 24 at the tip of the mounting pin 20 is press fitted into the stepped first hole 16. As a result, the locking projection 24 cannot return through the small-diameter portion of first hole 16. This eliminates the shortcoming of the prior art wherein the mounting pin 20 slips out of the buckle body 10.
The other prior-art example of a strap buckle has been illustrated in Japanese Utility Model Application Laid-Open No. 58-80021, such as shown in FIGS. 7 to 9.
This buckle includes a C-shaped buckle body 38 made of synthetic resin and first and second mounting pins 44, 46 formed at opposing surfaces of the leg portions 40, 42 of the buckle body 38, respectively. In this structure, the first and second mounting pins 44, 46 are provided with locking portion 48 as same as the locking projection 24 and neck portion 22 of the above-mentioned prior art and locking hole 50 having a configuration fitting the locking portion 48, respectively. The first and second mounting pins 44, 46 are press fitted by pressing the locking portion 48 into the locking hole 50.
In the structure of the former prior art shown in FIGS. 5 and 6, the metallic mounting pin 20 is required in addition to the buckle body 10, and also the mounting pin 20 must be formed to include the large-diameter head 26. Since a pin blank the diameter of which is larger than that of the head 26 must be cut away in order to make the metallic mounting pin 20 having the large-diameter head 26, costs are high and making the metallic mounting pin 20 is not suitable for mass production.
Further, in the structure of the latter prior art shown in FIGS. 7, 8 and 9, it is necessary to make the diameter of the locking portion 48 and locking hole 50 into larger diameter for taking sufficient strength. Therefore, the hole of the strap must be changed to larger hole, too. Furthermore, since the connecting portion of the first and second mounting pins 44, 46 is located to the middle of the whole mounting pin, the connecting portion directly receives the force added to the mounting pins. Thus, the connecting portion is in danger of separation always. Moreover, when the first and second mounting pins 44, 46 are inserted into the hole of the strap, the leg portions 40, 42 of the buckle body 38 must be spread apart largely, therefore, work efficiency of fabricating the buckle deteriorates.